One of the planet's greatest wildlife shows - the annual migration of more than a million wildebeest across east Africa's plains - is facing obliteration.

Scientists have warned that climatic uncertainty now threatens to turn the grasslands through which these great beasts trek each year into an uninhabitable desert. And the drought is blamed squarely on human activities: global warming triggered by carbon dioxide emissions from cars, planes and other factors. Intensive farming depleting fresh water supplies has also been blamed by climatologists.

'The migration is the greatest wildlife spectacle on Earth, and it would be catastrophic if we were no longer able to experience it,' said John Downer, who spent most of last year following the wildebeest on their great trek for a documentary.

The award-winning film-maker made use of remote cameras to track the movements of the wildebeest in places where it would have been too dangerous for humans to operate. One camera was hidden inside the head of a model hippo, enabling shots to be taken as the creatures made their perilous crossing of the Mara river, while a lens concealed inside a model dragonfly allowed close-up aerial views to be captured. Another camera was disguised as dung.

The migration - which has occurred without interruption for thousands of years - is one of the most extraordinary movements of animals on the planet. Around 1.5 million of these huge creatures trudge across Kenya and Tanzania in a vast 3,000km arc.

En route, the animals eat 7,000 tonnes of grass a day and drink enough water to fill five swimming pools. And around this time of year the migrating animals reach the Serengeti plain where they calve, triggering the biggest baby boom known. In three weeks, half a million wildebeest will be born.

But scientists fear this great cycle of reproduction could be wiped out if east Africa's drought gets much worse. In the past other mass migrations, in both Africa and Asia, have been disrupted and eradicated by humans who have fenced off land and diverted rivers and streams.

Ten years ago a million saiga antelopes migrated across central Asia. Now their population, has been reduced to 30,000, linked to habitat loss and the hunting of males for their horns to be used in traditional medicines. 'Such events seemed so indestructible but have been proved to be very fragile,' said Downer.

In recent years droughts have killed up to half a million migrating wildebeest in east Africa. If these droughts had continued by only a few more weeks, they would have killed off the region's entire wildebeest population. Now scientists fear that another major episode of water loss could trigger so many deaths as to leave no migrating wildebeest in east Africa. 'This is how serious the situation is,' said the WWF's eastern Africa regional office project manager, Doris Ombara.

One key threat comes from the Mara river, one of the region's principal waterways. Its flow has now dropped by more than half in the past few years. 'The river is dying,' said Ombara. On top of this, global warming has been linked to a drought that is gripping much of Africa - 'the strongest rainfall anomaly on the planet,' according to Dr Douglas Parker of Leeds University.

· Part One of 'Trek - Spy on the Wildebeest' will be shown at 8pm on BBC1 tonight.